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Wave Amplification & Frequency Transformation

  1. Jan 5, 2015 #1
    I understand the concepts behind the terms in the title; however, I have a question about how to transform the wave energy itself. I'm working on a science fair project that involves transforming sound energy into electrical energy--I understand this is not a very reasonable method of harvesting energy, but I figured it does have some use in the real world, and it's a unique experiment to conduct.

    My plan is to place a series of magnets on a non-magnetized string, and place a copper coil around it. When the sound waves travel along the wire, the magnets will displace longitudinally on the string, and this will form an electromotive force along the copper wire. To capture the sounds, I'm considering using a large cup-like object (similar to how cup telephones work) or even tying the string to a sub woofer.

    My goal is to maximize the longitudinal displacement of the magnets by increasing the amplitude of the waves. Since there is a definite amount of energy going in to the system, I can not simply increase the amplitude without sacrificing the frequency of the waves. I was wondering if there was any way to modify the frequency so that more of the energy goes to amplifying the sound waves as they travel across the string. Perhaps I could find a way to send certain frequencies so that constructive interference of the waves occurs. I've also considered using resonance amplification by placing two objects near each other (not sure how I should do this) with sound waves coming from one object and being manipulated via the other in order to increase the amplitude.

    If you have any tips on how to do this, how to have the tension on the string, what kind of magnets to use, how many magnets, etc., please let me know. Also, if you have any modifications for my projects, I will consider each idea. Finally, if you know of a source that contains valuable information of this topic, feel free to post it, and I'll read it for further consideration. Thanks!
  2. jcsd
  3. Jan 5, 2015 #2


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    Hi and welcome.
    When you get down to it, you are proposing an alternative design of microphone - (sound in / electrical power out). There are many alternative designs of transducers that will do this and they all have their own efficiency spec. What you propose appears to be a bit like an array of 'Ribbon Microphones', with their outputs coupled together. As with all such devices, there is a 'travelling wave' that interacts with a series of transducers. To get the outputs to augment each other, you need to introduce a progressive phase shift, so that all the outputs are in phase. This may be feasible but it does sound like hard work.
    It really depends what the bottom line is for your requirements of this exercise. If the point of the exercise is to 'do it this way' then that's fair enough but it could be (needlessly) difficult if you just want to design a form of transducer.

    I seem to gather that you want the magnets to vibrate(?). to get a magnet, with several grams of mass, to vibrate with a measurable amplitude, you will need very high forces. Most sound transducers are designed to have extremely light moving parts because that's the only way to get any usable level of electrical energy out.
    You should be aware that this is an Engineering Project and, in Engineering, the Numbers Count.
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